The present invention relates to a biquad type notch filter acting as an active filter and having a deep notch characteristic.
A notch filter is a band elimination filter or a band stop filter, for attenuating or damping a part of a frequency band in a pass-band. There are many types of active filters, however biquad type filters have an easily adjustable characteristic.
A normal type biquad circuit constitutes a first stage as an add-inverse low pass circuit, a second stage as an integrate inverse circuit, a third stage as an inverse circuit, and a feed back circuit from the third stage output to the first stage input. When an output of a notch filter is acquired, it is well known to be obtained using another outer adder weighing three signals, the first stage output and the second stage output or the third stage output, and adding them.
When Q is changed by a notch filter of this type, a gain of the first stage is changed in proportion to Q and simultaneously it may change a weight of a signal added from the first stage output to the outer adder. However, it is troublesome in that the operating level of a following stage circuit is high.
To avoid such trouble, another type of biquad circuit is constituted wherein a first stage is an add-inverse circuit, a second stage is an add-inverse circuit, a third stage is an integrate-inverse circuit and a fourth stage is an integrate-inverse circuit, and feed back circuits are from the third stage output to the first stage input and from the fourth stage output to the second stage input, respectively. When Q changes in this type of biquad circuit, only weight of signal is added an adder of from the first stage output to the second stage input, simultaneously it is changed in proportionate to Q. By this way, not only the circuit is easier than the former circuit is that gains of two stages must be changed, but the latter circuit has no trouble about operating level is high in proportionate to Q. However, the biquad circuit is that the first stage output is notch filter output. When Q enlarges in the circuit, a real operational amplifier (hereinafter, abbreviated xe2x80x9cOP amp.xe2x80x9d) is different from ideal model of this kind and a gain-bandwidth product (GB product) is a finite. Therefore, in many case, a heavy or deep notch is not obtained. When GB product seeks to be enlarged, it is difficult to be given a wide band and a high gain OP amp.
Aim of the present invention provides, according to afore-mentioned, in a biquad notch filter having double feed back circuits, when an OP amp is used. not so large GB product, constituted circuit is obtained having an enough deep notch characteristics.
In view of the foregoing problems, it is an object of the present invention to provide a biquad notch filter comprising:
a first and a second stages composing an inverse amplifier and a third and a forth stages composing an inverse integrator,
a feed back circuit connected from an output of the inverse integrator of the third stage to the inverse amplifier input of the first stage and from an output of the inverse integrator of the forth stage to the inverse amplifier input of the second stage, respectively,
said inverse amplifier composed of an operational amplifier and a feed back resistor between input and output of the amplifier,
said inverse integrator composed of an operational amplifier and a feed back capacitor between input and output of the amplifier,
empowered output of the inverse amplifier of the first stage being a filter output,
a reactance element connected to said feed back resistor of said second stage corresponding to a gain and cut off frequency of said inverse amplifier constituted as an impedance element, and obtaining a deep notch characteristic.
The biquad notch filter is preferred wherein the impedance element comprises an inductor connected in series to a feed back resistor of an inverse amplifier of said second stage.
The biquad notch filter is preferred wherein the impedance element comprises a capacitor connected in parallel to a feed back resistor of an inverse amplifier of said second stage.
The biquad notch filter is preferred wherein the impedance element comprises a capacitor connected to a feed back resistor in parallel and an inductor is connected to them in series.